There is a growing market demand for inexpensive air quality monitors for both research and personal health. Airborne particulate matter is among the deadliest forms of air pollution. The risk of lung cancer is greatly increased by the concentration of particulate matter below PM10. Asthma, cardiovascular disease, respiratory diseases and birth defects have also been associated with increases in airborne particulate matter concentration. In addition, these conditions are a detriment to the economy, resulting in thousands of workers on sick leave per day and billions of dollars straining health care systems around the world. Environmental researchers often do not have the budget to purchase multiple devices to develop data maps in order to monitor these adverse environmental conditions.
Airborne particulate matter can be measured gravimetrically to determine the mass concentration of matter in aerosol. However, this method is time consuming and often requires manual procedures. More recently, particulate matter has been measured with a Light Scattering Aerosol Spectrometer, or LSAS. These sensors count and size particles individually, respond quickly to changing environmental conditions, and can continuously monitor conditions for months without user intervention.
Typically, a LSAS works by drawing a sample of air through a beam of light. The beam of light is scattered due to the particles entrained in the sample of air. Optical collection systems direct the scattered light to a photodiode, which in turn converts the collected light into current that is then amplified into an analog voltage signal. The voltage signal is typically a pulse, where the pulse width and amplitude are proportional to the light intensity and particle diameter. The particle size, incident light, and other physical characteristics may be determined from this pulse. The concentration of particles entrained in the sample of air may also be determined by analyzing the pulses over time.
An inlet is typically used to draw a sample of air through the sensor. It may take the form of a nozzle or jet with either a round or rectangular profile. The round inlet provides a larger cross-section and requires a lower vacuum than the rectangular profile, resulting in lower power consumption. Although the round inlet is simpler to implement, the circular airflow has reduced uniformity and higher variations in light intensity across the intersection of the airflow and light beam. The rectangular profile provides better particle resolution because the flattened and wide airflow moves at a fairly uniform velocity across its area and intersects the light beam at the most intense and uniform region. However, the rectangular profile is more complex and expensive to manufacture.
As a particle passes through the laser, light is reflected and focused by the collection optics onto a photodiode. One difficulty in collection optics is the dependency of scattered light direction on particle size. Ideally, the sensor assembly captures all light and focuses it on to the photodiode while removing any unwanted light. Smaller particles typically scatter in the forward direction, whereas larger particles scatter at backward and right angles. If particles are significantly smaller than the cross-section of the light beam, they may not generate a high enough pulse to be distinguishable from signal noise. Border zone error occurs when particles straddle the optical border of the sensing zone, resulting in only a fraction of the light to be scattered. Coincidence error can occur with high particle number concentrations, where two or more particles are simultaneously present in the sensing zone.
Another difficulty lies in transport losses in the sampling tubing. The sampling system is part of a measurement chain and follows aerosol extraction, transport, and processing; the quality of the overall measurement is determined by the weakest element of this chain. The aerosol particles in the transport tubing can be affected by diffusion, sedimentation, inertia, condensation effects, and aggregation or coagulation. Aggregation or coagulation is a function of the collision and adhesion of particles. This results in a larger particle diameter and a smaller particle number concentration with constant mass concentration.
Current LSAS sensors are often complex and difficult to manufacture, and airborne particulate monitoring using them can be expensive, laborious, or inconvenient. In addition, airborne particulate matter has a strong link to lung cancer and cardiovascular disease. Accordingly, this results in a need for a low cost, effective method of monitoring and tracking environmental conditions.